54 THE PROTOZOA 



of arrangement form, as it were, a furry covering to the body ; and 

 their movements are different from those of flagella. A cilium 

 performs simple regular movements of alternate contraction and 

 relaxation, whereby it is first bent like a bow, with a slight spiral twist 

 (Schuberg, 44), and then becomes straightened out again ; from 

 this it may be inferred that the contractile substance is developed 

 mainly on one side of the elastic axis on that side, namely, which 

 becomes concave during contraction instead of ensheathing the 

 axis completely, as in most flagella. Then the bending of the 

 cilium would be the result of active contractility, acting against-* 

 the elasticity of the axis, which is operative in causing the 

 cilium to straighten out again when the contractile substance is 

 relaxed. 



Cilia are usually implanted in rows on the surface of the body, 

 and their movements are co-ordinated in such a way that the con- 

 traction or, as it may be better termed, the pulsation of a given 

 cilium takes place slightly after the one in front of it, and before the 

 one behind it (Fig. 26). On the other hand, the neighbouring cilia 

 of adjacent rows pulsate in unison ; consequently, when a ciliated 



//I 



FIG. 26. Diagram of ciliary movement, representing the successive phases of 

 contraction and expansion in a row of cilia. After Verworn. 



surface is seen from above with sufficient magnification, the move- 

 ments of the cilia produce an optical effect similar to that seen in a 

 cornfield when the wind blowing across it gives rise to an appearance 

 of waves following each other in a continuous succession. When, 

 however, a row of cilia is seen in side-view, the successive beats of 

 the cilia may produce the illusion of a rotating wheel ; hence the 

 origin of such names as Rotifer, Trochophore, etc., applied to 

 Metazoan organisms bearing rings or girdles of stout cilia. 



In spite of the apparent differences between cilia and flagella, 

 there is no difficulty in regarding cilia as derived ancestrally from 

 flagella by a process of modification and specialization in structure, 

 movement, number, arrangement, and co-ordination. Like pseudo- 

 podia and flagella, cilia may serve both for locomotion and food- 

 capture. In many cases the. cilia specialized for these two functions 

 may be sharply distinct ; the food-capturing cilia, found in connec- 

 tion with the mouth and the peristomial region, are commonly 

 much longer than the locomotor cilia, and show the tendency to 

 form fusions presently to be described. In sedentary forms loco 

 motor cilia may be absent in the ordinary state of the animal, and 

 only developed temporarily during motile phases. On the other 



